1. Field of the Invention
The invention relates to a capacitive moisture sensor with a dielectric comprising a moisture-sensitive organic polymer which is mounted between two electrodes. The invention also relates to a method of preparing this type of a capacitive moisture sensor.
2. Description of Related Art
The detection or determination of moisture in gases, in particular in the air, plays an important role in all air-conditioning matters, such as in air-conditioning buildings as well as in refrigeration or green houses, and in dehumidifying processes. Reproducible operating moisture sensors or detecting devices, which should be cost effective, should guarantee energy-saving air conditioning or dehumidification in connection with a corresponding, automatic measuring and control system.
Capacitive moisture sensors, in which the relative changes in capacitance .DELTA.C/C steadily increase with the amount of water absorbed in the dielectric and therefore also serve as moisture measuring parameters, are particularly suited for measuring the relative atmospheric humidity, which is important for processing techniques. In connection with this, the moisture sensors must have a series of important properties, namely quick response time, little or no temperature sensitivity, no long-term drift, insensitivity to flowing gases, resistance to corrosion in particular to aggressive media, and low cross-sensitivity in particular to organic solvents Moreover, the sensors must deliver electronically measurable signals which are able to be processed in a simple manner by modern microcomputer systems. For reasons of circuit engineering, a linear correlation between the absorbed moisture and the sensor signal is advantageous.
Cost effective, capacitive moisture sensors can be realized e.g. with thin, water-permeable metal electrodes and a moisture sensitive or responsive organic polymer, whose dielectric constant changes characteristically with the absorbed moisture. Such a moisture sensor is known e.g. from DE-OS 28 48 034. The advantages of moisture sensors of the said type are above all a measurement, i.e., moisture determination, which is independent from the flow rate of the surrounding medium; a uniform measuring responsiveness; a low maintenance cost; and a quick response rate (c.f. e.g.: K. Carr-Brion, "Moisture sensors in Process Control", Elsevier Applied Science Publishers, London and New York, 1986, pp 22 to 24). Incidentally, in addition to the moisture-dependent measurement of changes of capacitance (also c.f.: "Sensors and Actuators", Vol. 12 (1987), pp 291 to 296, as well as: U.S. Pat. No. 4,164,868), measurements of the electrical resistance can also be drawn upon as a measure for the relative atmospheric humidity (c.f. "Sensors and Actuators", Vol. 8 (1985), pp 23 to 28, as well as: "Proc. 3rd Int. Conf. Solid-State Sensors and Actuators (Transducers '85), June 14-15, 1985", pp 210 to 212) and changes in impedance (c.f.: "Sensors and Actuators", Vol. 13 (1988), pp 243 to 250).
Various materials are already known as moisturesensitive polymers for moisture sensors, e.g.: cellulose acetate butyrate ("IEEE Transactions on Components, Hybrids, and Manufacturing Technology", Vol. CHMT-2 (1979), No. 3, pp 321 to 323, as well as: "Sensors and Actuators", Vol. 12 (1987), pp 291 to 296);
polyimide ("Proc. 3rd Int. Conf. Solid-State Sensors and Actuators (Transducers ,85), June 11-14, 1985", pp 217 to 220);
cross-linked copolymers of styrene sulfonate and polyvinylchloride ("Sensors and Actuators", Vol. 8 (1985), pp 23 to 28);
copolymers of ionic monomers, like the sodium salt from styrene-4-sulfonic acid and methacrylic acid, or non-ionic monomers like styrol and (meth)acrylic acid methylester ("Proc. 3rd Int. Conf. Solid-State Sensors and Actuators (Transducers ,85), June 11-14, 1985" pp 210 to 212);
graft copolymers of polytetrafluoroethylene and styrene or 4-vinylpyridine with acid or alkaline groups ("Proc. 3rd Int. Conf. Solid-State Sensors and Actuators (Transducers '85), June 11-14, 1985" pp 213 to 216); and quaternized 4-vinylpyridine styrene copolymers and poly-4-vinylpyridine which is partially quaternized or crosslinked with dibromobutane ("Sensors and Actuators", Vol. 13 (1988), pp 243 to 250).
The primary disadvantages of moisture sensors with polymer materials of the aforesaid type are a limited measuring range, a strong temperature sensitivity, the appearance of hysteresis and poor long-term stability. Moreover, these types of materials are often accessable only as foils, whose available thickness does not permit a desirable, quick response time.
It is an object of the invention to provide a capacitive moisture sensor of the aforesaid type with a dielectric comprising a moisture sensitive organic polymer between a pair of electrodes.
It is a further object of the invention to provide such a capacitive moisture sensor which has a high, long-term stability and which fulfills all the requirements of such sensors to a high degree.
It is a further object of the invention to provide such a capacitive moisture sensor wherein the least possible amount of long-term drift results and wherein a structuring of the dielectric should be possible.